This is not an engineering exercise, but rather a statement of experienceDrag racing truly requires a great compromise, no rather a balance, betweentire size and rear axle final gear ratio.If your car is to be for exclusively drag racing, you need to optimize thiscombination. As you approach 400 HP and proceed to 600/700 HP range, tire size will bethe most limiting factor for traction and 60 foot times. When originallybuilding my XR4, I chose 28 X 10 tire size considering that performancewould approach the limits of that tire. (Actually 700 HP does require moretire).If the spring rates can be increased sufficiently and half shafts controlledso the tires will hook, the strength of the ring and pinion gears or otherparts in the rear end will become a problem. I have known drag racers instock classes run really small axles because it was required, and theyactually changed axle shafts between rounds. (the splined end of the shaftwas the weak link).Additionally, ratio availability for the rear is quite limited, to a pointthat you would have to run a much smaller tire and be traction limited.This trade off in traction and mechanical advantage is tricky. As a pointof reference, the combination to go 208 MPH is essentially going 7000 RPMthru the mile with 2.75:1 gears. If you have a package similar to make thepower you suggest, Horsepower at that engine speed is still good. we couldnot measure power above 6700 rpm because of running out of dyno, but theengine was still pulling at that point. Engine parts used were Esslinger aluminum head, (old round exhaust ports),My own inlet system (EFI) using Esslingers casting for methanol injection, avalve train good to 8500 RPM with .685" Net lift Esslinger roller cam. (this was a joint project with one of my Ford friends). It worked well. Aturbo that is about 4 times the air flow of the production unit,
The early combination that went 11.53 ET was iron head, ice water charge aircooler, Lunatti .500 cam, 5 gears used, 5.88:1 rear with the above mentioned28"X10" Goodyear slicks. I think that I was making less than 400 HP then,but ran well because of the good combination of hook and power. Later weused just 4 speeds and 4.56:1 gears with a 3.35 1st gear, but was not reallymaking serious attempts to ET or MPH, it was just a way to test. I used aT5 that was pro shifted, or modified by Joe Liberty to do the typical dragracing pedal on the floor clutchless shifts. I'm going into detail likethis because you need to know what you are getting into relative to makingan XR4 go really quickly. Now my car was at the time around 3200 lbs. Ifyou did a massive weight reduction program and made the cage as light aspossible, you may be around 2600 lbs, and be able to get away with theproduction type of axle, but overall, I think it would pay to do the liveaxle swap in the beginning. I'm not even sure that the Quaife will take theviolence of repeated launches at 7000 RPM that it takes to seriously launch.I fear you would be breaking parts on a regular basis and if you are like meI HATE to fail parts. Its embarrassing and expensive.Sorry if this is contrary to what you want to hear, but if you want aconsistent 11 second or quicker car this is what I think you need.
NOW on the other hand:About a year ago I was in discussion with someone from Calif that wasinvolved some years ago making an IRS vehicle do well at the drags. Ibelieve this effort was somewhat successful, but I cannot remember who itis. (CRS syndrome).Asking the questions again of the list may draw him out. For that reason Iwill post this note to the lists.